The present invention relates to a method for producing an exchanger comprising a number of hollow exchanger tubes.
Such an exchanger can be a heat exchanger, a substance exchanger, or separator. More particular the invention relates to an exchanger to be used at elevated temperature. Because of the conditions of use, ceramic material to provide sufficient service life is used in the art. As example hollow fibre membrane tubes with small tube diameter are mentioned. At higher temperature levels polymer membranes are no longer usable. Advantages of ceramic membranes are high temperature resistivity, mechanical stability and normally those membranes are not sensitive to chemical attack in, for example, corrosive surroundings. An example for a method to produce a ceramic hollow fibre membrane allowing for high fluxes is disclosed in EP-0 693 961 A1.
In practical use only tubes are not sufficient. They have to be incorporated in a module to make practical use thereof. Generally, such a module comprises an enclosure and a pipe plate for receiving the ends of the hollow tubes as well as possibly further plates to distribute flows both in the hollow tubes and around those tubes.
Up till now it was technically not feasible to produce in an economical way pipe plates which could withstand relative high temperatures and in which also some mechanical loading is observed and which can be scaled up easily.
It has been shown that many gas separation processes can be conducted at high temperature with ceramic membranes. An example is the xe2x80x98Knudsenxe2x80x99 separation with gamma-Al2O3 membranes, separation of hydrogen from a mixture of gases with silica membranes (300-500xc2x0 C.), isomer separation with zeolite membranes (xc2x1500xc2x0 C. max.) and electrochemical separation of oxygen from air with mixed ionic-electronic conducting materials (800-1000xc2x0 C.). These are only examples for which high temperature exchangers could be used, more in particular the hollow fibre membrane tubes. In order to scale-up these processes high temperature resistant membrane modules allowing for high fluxes have to become available for relative low prices. JP-A-61 004509 discloses a heat exchanger comprising a bundle of thin glass membrane tubes. Because at a relatively low temperature glass material will lose its strength and start yielding, the use of a heat exchanger made according to the Japanese patent application is restricted to a relatively low temperature. Furthermore, additional measures are necessary to complete the heat exchanger. The glass tubes are connected to the pipe plates by introducing them in a mixture of glass and ceramics. Through heating, a sintering and bonding step would occur. Glass will function as a binder. However, if a higher temperature use is desired, it is no longer possible to use glass as a binder.
The invention aims to provide a method for producing such a module, a pipe plate respectively for an exchanger to be used at elevated temperatures having sufficient strength and manufactured in such a way that it is economically feasible. Of course, there should be a perfect sealing between the tubes and the pipe plate.
The tubes to be used according to the invention can be the hollow fibre membranes as described above. However, it should be noted that for other applications hollow ceramic tubes (i.e. having a larger diameter) or metallic tubes can be used. With the casting method according to the invention good sealing properties between the pipe plate and the tubes have been observed, whilst even after repeated heating and cooling down of the exchanger no cracks occurred. The method as described is economical to realise and provides an exchanger in which the pipe plate is no longer the restricting component for the temperature at which the exchanger can be used.
According to the invention sintering is obtained by using a slurry of a ceramic and at least one solvent, pouring the slurry in a mould which is able to absorb the solvent, i.e. removes the solvent before actual heating at elevated temperature before sintering starts. Because of that, in a controlled way positioning of the ceramic material can be obtained without a negative effect of gas bubbles, which would form during heating of the solvent, rising through a pipe plate. By sintering the enclosure to the pipe plate a mechanically stable and gas-tight structure is obtained.
The present invention deals with an economically attractive way to join simultaneously a large number of hollow tubes, more specifially ceramic hollow fibre membranes, to a substance exchanger, more specifially a ceramic membrane module with which liquid separation, gas permeation, gas separation and related processes at high temperature can be conducted. Furthermore, the invention is of importance at high-temperature heat exchanging processes and high-temperature ceramic or inorganic equipment in which leak-free ceramic/ceramic connections are required.
If the exchanger tubes are hollow fibre membranes, such ceramic membranes are first made as tubes or hollow fibres in the shape and condition in which they will be in the completed exchanger. This means that the desired pore size and porosity are already adjusted which can be effected by a heat treatment at elevated temperature, for example as described in EP- 0 693 961 (between 1200 and 1650xc2x0 C.). Ceramic membranes can be porous or (inert) gas tight and can comprise one or more (concentric) layers.
To obtain a possibility of introducing a fluid around the pipes, according to a preferred embodiment of the invention, there is also arranged a feed/discharge pipe in the pipe plate. To provide sealing, it is possible to provide a feed/discharge pipe while casting the slurry in a mould in, or onto which the exchanger tubes are positioned. However, it will be understood that such feed/discharge pipes can also be connected to any other part of the enclosure.
The enclosure according to the invention can comprise a ceramic material and can have any shape known in the art. Preferably, it is also placed on, or in the mould to provide sealing engagement between the pipe plate and the enclosure.
In order to realize a plenum for the hollow tubes an end plate has to be provided spaced from the pipe plate. Such an end plate can be produced with the same method as described above for preparing the pipe plate. Dependent from the slurry and its condition used for the pipe plate, it is possible that this cast and dried slurry for the pipe plate is still in green condition whilst the end plate is cast so that both the end plate and the pipe plate can be sintered in one step. Of course production in two sintering steps is possible.
The other end of the exchanger can be produced in the same way and also for this production either separate sintering steps can be used or one general sintering step. In the last case the green strength of the otherxe2x80x94opposedxe2x80x94pipe plate will be sufficient to allow handling of the exchanger to be built. Because the ceramic material resulting from the slurry after sintering has about the same thermal expansion coefficient as other components used in the exchanger (difference less than 5xc3x9710xe2x88x926Kxe2x88x921), thermal tensions in the exchanger are minimized. Also mechanical properties are sufficient either at high temperature and extreme process pressures. For the ceramic material used in the exchanger, use can be made of aluminum oxide, silicon carbide, silicon nitride, zirconium oxide, hydroxyapatite, perovskites and other substances.
In all these cases, the composition of the ceramic slurry is chosen in such a way that after casting, minimum shrinkage occurs during drying and sintering.
As described above, the hollow tubes used can be in finished condition at the moment the pipe plates are cast around them. However, it is also possible to subject the internals of the heat exchangers to a further conditioning before or after sintering. To that end a liquid or vapour can be entered in the related compartment which can provide a coating having the desired properties. It is also possible to introduce two separate liquids or vapours in the two exchanging compartments which will diffuse into each other and react only at their interface in order to obtain the desired properties. In case that the pipe plates and end plates are not gas-tight yet, it may be desirable to apply a gas-tight, leak-free coating on these plates. In this case the ceramic compact gives mechanical strength whereas the coating is responsible for gas-tightness. The components for obtaining coatings or further treating the material of the exchange components can be introduced as sol-gel layer and after filling the related compartments excess material is drained. It is also possible to condition only determined parts of a compartment by appropriate positioning of the related part of the exchanger at introduction of the material to be applied on the related component.
The invention also relates to an exchanger. Surprisingly it has been found that no further sealing is necessary under ordinary conditions to provide a sufficient type fit between the tubes and the pipe plate.
EP 0 093 612 a1 discloses a heat exchanger without enclosure wherein the tube ends are received in spaced apertured pipe plates. The area between two adjacent pipe plates is filled with a ceramic slurry and hardened. This means that a product after hardening is not self-supporting. EP 0 165 478 A1 shows a substance exchanger wherein the exchanger tubes are of cellulose material which are placed in a polyurethane end plate. This is a low temperature exchanger. EP 0 794 403 A2 shows a heat exchanger having tubes extending from one pipe plate in which further tubes are introduced. There is no opposite side plate and no enclosure extending between said pipe plate. The enclosure of the exchanger according to the invention can be both a ceramic and metallic material. If a metallic material is used, it should also have a coefficient of expansion approaching the coefficient of expansion of the ceramic material as indicated above. An example of a metallic material which can be used is a Fexe2x80x94Nixe2x80x94Co alloy known as VACON or DILVER P.
However, if further sealing is necessary, for example if relatively porous ceramic materials are used, the coating process as described above be applied.